Recent experimental results indicate that the ultimate carcinogenic forms of benzo(a)pyrene and related arylhydrocarbons are dihydrodiol oxide derivatives. These derivatives are known to be produced metabolically, by the sequential action of monooxygenase and epoxide hydrase activities. It has also been established that altering the relative activities of these two enzymes will alter the mutagenic properties of arylhydrocarbon metabolites. The epoxide hydrase enzyme activity appears to be subject to modification by the binding of specific compounds to effector sites. We have found, for example, that flavone, 7,8-benzoflavone, 9-fluorenone, and chalcone oxide will stimulate the epoxide hydrase catalyzed hydrolysis of styrene oxide. We propose continuing our investigations of the structure-activity relationships of stimulators of epoxide hydrase. Experiments to detect and identify an effector site on the enzyme will also be performed. Finally, we will attempt to determine how modification of the activity of epoxide hydrase within a tissue may moderate or potentiate the carcinogenicity of arylhydrocarbons such as benzo(a)pyrene.